The invention relates to a light switching device that is reversibly switchable between at least a first state of reflecting light and a second state, the second state being either a state of absorbing light or a transmissive state, said device comprising a stack of layers including a switchable layer of an optically switchable material which brings about a switch from the first state to the second state of the device in particular a material in which switching is obtained by changing a density of hydrogen.
U.S. Pat. No. 5,905,590 describes a switching device comprising a switching film including hydrides of magnesium with other trivalent metals. By exchange of hydrogen, the switching film can be reversibly switched from a transparent state to a mirror-like (fully reflecting or scattering) state with zero transmission via an intermediate black absorbing state. The switching film is comprised in a stack of layers, which is deposited on a transparent substrate. By virtue of the optical effect the device can be used as an optical switching element, for example as a variable beam splitter, optical shutter, and for controlling the illuminance or the shape of light beams in luminaires. The switching device can also be used for data storage and in optical computing, and in applications such as architectural glass, vision control glass, sunroofs and rear-view mirrors. By making a pattern in the switching film and providing the patterned switching film with transparent electrodes a thin display can be manufactured.
It is a problem with this type of devices that since the speed of the switching effect is determined by the transport of hydrogen, the device is relatively slow.
It is an object of the invention to provide a switching device, which has an improved speed. To this end, the invention provides a display device in accordance with claim 1.
The invention is based on the insight that on the one hand the amount of charge required to address a pixel is so large that this cannot be loaded to a pixel during a number of subsequent addressing periods while on the other hand the pixel is comparable to a rechargeable battery. In the switching mirror device H-ions diffuse from one H-containing layer to the other whilst an electric current flows around the device. The optical properties depend on the state of charge of this hydrogen battery. By changing the polarity of the applied voltage the H-ions will flow back.
By introducing, according to the invention, for each pixel element a series connection of the pixel element and a current source to charge the pixel element and further means for resetting at least part of the pixel elements to a defined state, a large current can be introduced in both directions, allowing fast switching of the pixels.
In a first embodiment of the invention the common point of the pixel element is connected via a switch to a reset voltage. Reset is obtained by closing the switch.
In a preferred embodiment of the invention the series connection of the pixel element and the current source is provided between voltage connections and the means for resetting comprise means to switch one of the voltage connections between at least two voltages having different signs with respect to the other voltage connection (symmetric devices). In this way the extra switch for reset can be dispensed with, which in the transmissive mode leads to a higher aperture.
For asymmetric devices it may also be possible to switch between two different amplitudes.
Preferably the current source comprises a capacitor element connected to a control connection of a transistor.